The present invention relates generally to microelectromechanical system (MEMS) devices and, more particularly, to a method of fabricating a MEMS device having a release etch stop layer.
MEMS devices are employed as actuators and sensors in a wide variety of applications. Capacitive-sensing MEMS devices, for example, are now commonly employed in many different electronic devices to sense acceleration, vibration, device orientation, and other inertia-related parameters. Such MEMS devices operate by sensing changes in capacitance between electrodes in a transducer structure. The transducer structure may include, for example, a number of stationary electrodes or “fingers” interposed with and spaced apart from a number of movable electrodes or “fingers” in a comb-type arrangement. The movable electrodes are rigidly joined to a larger movable structure commonly referred to as a “proof mass,” which is resiliently suspended over an underlying substrate. During operation of an exemplary MEMS device, a voltage differential may be applied across the stationary or movable electrodes. As the proof structure moves in response to acceleration of the MEMS device, the movable electrodes are displaced with respect to the fixed electrodes and the capacitances between the electrodes vary accordingly. By monitoring these capacitances, the acceleration or other movement of the MEMS device can be determined.
At an intermediate juncture during fabrication, the partially-fabricated MEMS device may include an unpatterned transducer layer overlying a number of sacrificial oxide layers. In one known fabrication method, which may be referred to as an engineered Silicon-On-Insulator (“eSOI”) based MEMS process, separately-fabricated transducer and substrate wafers are bonded together to produce the MEMS device. If such an eSOI based MEMS process is utilized to produce the MEMS device, one or more sacrificial oxide layers may initially be formed on a substrate wafer (also commonly referred to as the “handle wafer”), which is bonded to an additional sacrificial oxide layer formed over the unpatterned transducer layer of a separate transducer wafer. The transducer wafer may also contain a patterned interconnect layer underlying the unpatterned transducer layer and interspersed with the sacrificial oxide layers. After bonding of the substrate and transducer wafers, the unpatterned transducer layer is lithographically patterned to define the primary transducer structure including the proof mass. The bulk of the sacrificial oxide layers may then be removed through the openings in the patterned transducer layer to mechanically release the proof mass. Portions of sacrificial oxide layers may be purposefully left intact, however, to provide anchor structures supporting the patterned transducer layer through the interconnect layer. Removal of the sacrificial oxide layer is commonly accomplished utilizing a release etch process wherein the sacrificial oxide layers are exposed to an oxide-selective etchant, such as a hydrogen fluoride-based etchant, for a predetermined period of time. The duration of release etch is carefully controlled to ensure complete removal of the portions of the sacrificial oxide layers directly underlying the transducer structure, while also minimizing undesired material loss from the portions of the sacrificial oxide layers forming the anchor structures.
For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction and may omit depiction, descriptions, and details of well-known features and techniques to avoid unnecessarily obscuring the exemplary and non-limiting embodiments of the invention described in the subsequent Detailed Description. It should further be understood that features or elements appearing in the accompanying figures are not necessarily drawn to scale unless otherwise stated. For example, the dimensions of certain elements or regions in the figures may be exaggerated relative to other elements or regions to improve understanding of embodiments of the invention.